How to call a codebehind function from javascript

Question

Hi, i need some help calling a function in my .ascx.cs of a visualwebpart for SP2013 from JS.
The idea is to execute a function when a datepicker date is selected
<script type="text/javascript" src="../_layouts/15/jQuery/jquery.min.js"></script>
<script type="text/javascript" src="../_layouts/15/jQuery/bootstrap.min.js"></script>
<script type="text/javascript" src="../_layouts/15/jQuery/bootstrap-datetimepicker.min.js"></script>
<script type="text/javascript">
$(function () {
$('#datetimepicker4').datetimepicker()
.on('changeDate', function callWebMethod() {
$.ajax({
type: "POST",
url: "Visualizador.ascx/Testing",
data: "{}",
contentType: "application/json; charset=utf-8",
dataType: "json",
success: function (data) {
alert("OK");
error: function (jqXHR, textStatus, errorThrown) {
alert("Error");
</script>
The problem is that it always return the "Error" alert.
The functions name is "Testing" (in Visualizador.ascx.cs), the project name is "BuscadorCumple" and the VisualWebPart is "Visualizador"
I´ve tried with the PageMethods but i can´t add a ScriptManager because the MasterPage of the site has already one, and only one ScriptManager is allowed. So if i could set the EnablePageMethods to true it would be great (to use the PageMethods), or just
try to fix the js function from above.
THANKS IN ADVANCE
------------------------------------------------Update----------------------------------------------
Here is the method i´m using to test it, inside Visualizador.ascx.cs
[WebMethod]
public static void Testing()
Response.Write("<script>alert('HW')</script>");

Answer 1

Methiod should be like this
[WebMethod]
  public static
string GetData()
    string str="hi";   
return str;
Calling is like this
$.ajax({
type: "POST",
url: "PageName.aspx/MethodName(GetData)",
data: "{}",
contentType: "application/json; charset=utf-8",
dataType: "json",
success: function(msg) {
// Do something.